Photodiode

02/05/09 http://en.wikipedia.org/wiki/Photodiode #1
Photodiode
From Wikipedia, the free encyclopedia
A photodiode is a type of photodetector capable of converting light
into either current or voltage, depending upon the mode of
operation. [1]
Photodiodes are similar to regular semiconductor diodes except that
they may be either exposed (to detect vacuum UV or Xrays) or
packaged with a window or optical fibre connection to allow light to
reach the sensitive part of the device. Many diodes designed for use
specifically as a photodiode will also use a PIN junction rather than
the typical PN junction.
Contents
Polarity
1 Polarity
2 Principle of operation
2.1 Photovoltaic mode
2.2 Photoconductive mode
2.3 Other modes of operation
3 Materials
4 Features
5 Applications
5.1 Comparison with photomultipliers
5.2 PN vs. PIN Photodiodes
6 Photodiode array
7 See also
8 References
9 External links
Some photodiodes will look like the picture to the right, that is, similar to a light emitting diode.
They will have two leads, or wires, coming from the bottom. The shorter end of the two is the
cathode, while the longer end is the anode. See below for a schematic drawing of the anode and
cathode side. Under forward bias, conventional current will pass from the anode to the cathode,
following the arrow in the symbol. Photocurrent flows in the opposite direction.
Principle of operation
A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the
diode, it excites an electron, thereby creating a mobile electron and a positively charged electron
hole. If the absorption occurs in the junction's depletion region, or one diffusion length away from
it, these carriers are swept from the junction by the builtin field of the depletion region. Thus
holes move toward the anode, and electrons toward the cathode, and a
photocurrent is produced.
Photovoltaic mode
When used in zero bias or photovoltaic mode, the flow of photocurrent
out of the device is restricted and a voltage builds up. The diode
becomes forward biased and "dark current" begins to flow across the
junction in the direction opposite to the photocurrent. This mode is
responsible for the photovoltaic effect, which is the basis for solar
cells—in fact, a solar cell is just an array of large area photodiodes.
Photodetector from a CDROM Drive. Visible are 3
photodiodes.
A photodiode
Photodiode schematic symbol

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Photoconductive mode
In this mode the diode is often (but not always) reverse biased. This increases the width of the depletion layer, which
decreases the junction's capacitance resulting in faster response times. The reverse bias induces only a small amount
of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same.
The photocurrent is linearly proportional to the illuminance.[1]
(http://hyperphysics.phyastr.gsu.edu/hbase/Electronic/photdet.html)
Although this mode is faster, the photovoltaic mode tends to exhibit less electronic noise. (The leakage current of a
good PIN diode is so low – < 1nA – that the Johnson–Nyquist noise of the load resistance in a typical circuit often
dominates.)
Other modes of operation
Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher
reverse bias. This allows each photogenerated carrier to be multiplied by avalanche breakdown, resulting in internal
gain within the photodiode, which increases the effective responsivity of the device.
Phototransistors also consist of a photodiode with internal gain. A phototransistor is in essence nothing more than a
bipolar transistor that is encased in a transparent case so that light can reach the basecollector junction. The
electrons that are generated by photons in the basecollector junction are injected into the base, and this photodiode
current is amplified by the transistor's current gain β (or hfe). Note that while phototransistors have a higher
responsivity for light they are not able to detect low levels of light any better than photodiodes. Phototransistors also
have slower response times.
Materials
The material used to make a photodiode is critical to defining its properties, because only photons with sufficient
energy to excite electrons across the material's bandgap will produce significant photocurrents.
Materials commonly used to produce photodiodes include:
Material Wavelength range (nm)
Silicon 190–1100
Germanium 400–1700
Indium gallium arsenide 800–2600
Lead sulfide

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(NEP) The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz
bandwidth. The related characteristic detectivity (D) is the inverse of NEP, 1/NEP; and the specific detectivity (
) is the detectivity normalized to the area (A) of the photodetector, . The NEP is roughly the
minimum detectable input power of a photodiode.
When a photodiode is used in an optical communication system, these parameters contribute to the sensitivity of the
optical receiver, which is the minimum input power required for the receiver to achieve a specified bit error ratio.
Applications
PN photodiodes are used in similar applications to other photodetectors, such as photoconductors, chargecoupled
devices, and photomultiplier tubes.
Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the
receivers for remote controls in VCRs and televisions.
In other consumer items such as camera light meters, clock radios (the ones that dim the display when it's dark) and
street lights, photoconductors are often used rather than photodiodes, although in principle either could be used.
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a
better, more linear response than photoconductors.
They are also widely used in various medical applications, such as detectors for computed tomography (coupled with
scintillators) or instruments to analyze samples (immunoassay). They are also used in blood gas monitors.
PIN diodes are much faster and more sensitive than ordinary pn junction diodes, and hence are often used for optical
communications and in lighting regulation.
PN photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed,
avalanche photodiodes, intensified chargecoupled devices or photomultiplier tubes are used for applications such as
astronomy, spectroscopy, night vision equipment and laser rangefinding.
Comparison with photomultipliers
Advantages compared to photomultipliers:
1. Excellent linearity of output current as a function of incident light
2. Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other semiconductor materials
3. Low noise
4. Ruggedized to mechanical stress
5. Low cost
6. Compact and light weight
7. Long lifetime
8. High quantum efficiency, typically 80%
9. No high voltage required
Disadvantages compared to photomultipliers:
1. Small area
2. No internal gain (except avalanche photodiodes, but their gain is typically 10²–10³ compared to up to 108 for
the photomultiplier)
3. Much lower overall sensitivity
4. Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic
circuits
5. Response time for many designs is slower
PN vs. PIN Photodiodes
1. Due to the intrinsic layer, a PIN photodiode must be reverse biased (Vr). The Vr increases the depletion region
allowing a larger volume for electronhole pair production, and reduces the capacitance thereby increasing the
bandwidth.
2. The Vr also introduces noise current, which reduces the S/N ratio. Therefore, a reverse bias is recommended for
higher bandwidth applications and/or applications where a wide dynamic range is required.
3. A PN photodiode is more suitable for lower light applications because it allows for unbiased operation.

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Photodiode array
Hundreds or thousands (up to 2048) photodiodes of typical sensitive area 0.025mmx1mm each arranged as a
onedimensional array, which can be used as a position sensor. One advantage of photodiode arrays (PDAs) is that
they allow for high speed parallel read out since the driving electronics may not be built in like a traditional CMOS or
CCD sensor.
See also
Electronics
Band gap
Infrared
Optoelectronics
Optoisolator
Semiconductor device
Solar cell
Avalanche photodiode
Transducer
LEDs as Photodiode Light Sensors
Light meter
Ambientlight meter
References
This article contains material from the Federal Standard 1037C, which, as a work of the United States Government, is
in the public domain.
1. ^ International Union of Pure and Applied Chemistry. "Photodiode (http://goldbook.iupac.org/P04598.html) ". Compendium of Chemical
Terminology Internet edition.
Gowar, John, Optical Communication Systems, 2 ed., PrenticeHall, Hempstead UK, 1993 (ISBN 0136387276)
External links
Technical Information Hamamatsu Photonics
(http://sales.hamamatsu.com/assets/html/ssd/siphotodiode/index.htm)
Using the Photodiode to convert the PC to a Light Intensity Logger (http://www.emant.com/324003.page)
Design Fundamentals for Phototransistor Circuits (http://www.fairchildsemi.com/an/AN/AN3005.pdf)
Working principles of photodiodes (http://ecewww.colorado.edu/~bart/book/book/chapter4/ch4_7.htm)
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